As a material light in weight and high in strength, an FRP is paid attention to in various industrial fields, and in particular, a carbon fiber reinforced plastic (CFRP) is paid attention to from the viewpoint of its excellent mechanical properties, etc. Especially, in the air plane field, the application part of an FRP is being enlarged, and it is a material expected with a further development in the future.
In a fiber reinforced plastic whose reinforcing fibers are continuous carbon fibers, although the mechanical properties in the plane of fiber axes are excellent, if an impact is partially applied to a CFRP plate, stacked with carbon fiber sheets, in its thickness direction, there is a case where a crack is generated between the layers in the CFRP plate by the impact and the layers are broken away from each other at the interlaminar portion, and therefore, various methods for strengthening the interlaminar portion have been proposed.
In an FRP using such a CFRP plate, as the typical production method thereof, usually a preform, in which reinforcing fiber substrates are stacked in a form of an FRP to be molded in advance, is used, but there are the following problems when a matrix resin is injected and the FRP is molded. Namely, since generally an impregnation speed of a matrix resin into a preform greatly depends on a porosity of the preform (a rate of pores in a preform), in a condition where a reinforcing fiber volume content (a bulk density) is high (so-called “near net shape”), the impregnation speed becomes low because of its small porosity. Further, even in a condition of same porosity, there is a characteristic in that the impregnation speed in the plate thickness direction is much lower than that in the direction in plane of reinforcing substrate. Therefore, in the injection molding of CFRP, particularly when a near net shape molding or a molding of a thick plate is carried out, because the impregnation speed into the preform is low, the cycle time for molding becomes long, there is a problem that the productivity becomes low, and as the worst case, there is a problem that the impregnation is not completed within a time of a pot life of the resin, and a non-impregnated portion is left in the molded product.
As the method for reinforcing the interlaminar portion between the substrate layers, as shown in Patent Document 1, there is a method for adding a resin material whose main component is a thermoplastic resin to the interlaminar portion between the reinforcing fiber layers in the preform, thereby increasing the strength of the interlaminar portion. Since this resin material has not only an advantage of reinforcing the interlaminar portion but also a function of bonding and fixing the reinforcing fiber substrates to each other in the preform, it is a very effective material also for the production of the preform.
With respect to the porosity of this preform, however, in comparison with a usual reinforcing fiber substrate provided with no resin material at a same reinforcing fiber volume content, because the porosity of the preform becomes small by an amount corresponding to the volume of the resin material in the preform, there is a problem that the impregnability is poor. Further, in a condition where the reinforcing fiber volume content (the bulk density) of the preform is particularly high, because there is a case where the interlaminar portion becomes thinnest, that is, becomes a film-like state even if any form of resin material is employed, there is a problem that the resin material present in the interlaminar portion obstructs the matrix resin flowing in the plate thickness direction to be impregnated into the reinforcing fiber layers. Namely, although the preform is very excellent in various mechanical properties, particularly in a case where a molding at a high reinforcing fiber volume content, that is, a molding at a so-called near net shape condition, is carried out, or in a case where a thick preform is molded, there is a problem that much time is required in the impregnation process of the matrix resin.
Further, as described in Patent Document 2, there is also a method wherein reinforcing fibers are raised by needle punching, the raised yarn fibers or short fiber webs are pressed into the portion between layers, and the shear strength and the antiplane strength of the interlaminar portion is increased by a condition where the layers are bridged to each other. In this method, the impregnability in the plate thickness direction is increased by the capillarity of the raised fibers, and the interlaminar strength can be increased by the anchor effect due to the raised fibers. In this method, however, because the raising is carried out by daring to cut a part of reinforcing fibers at the time of needle punching, the strength decreases by the amount corresponding to the cut fibers. Further, because a stress concentration is liable to occur at the punched portion, although there is an advantage for increasing the interlaminar strength, as its side effect, there is a problem that the basic mechanical properties such as a tensile strength and a compressive strength in one direction depending on the amount of reinforcing fibers are decreased.    Patent Document 1: JP-A-2003-80607    Patent Document 2: JP-A-2003-39429